This application is a continuation of International Patent Application No. PCT/EP2004/003867 filed on Apr. 13, 2004, designating the United States of America, the entire disclosure of which is incorporated herein by reference. Priority is claimed based on German Patent Application No. DE 103 21793.2 filed May 14, 2003.
This invention relates to a method for operating a bivalent internal combustion engine which can be operated under lean conditions at λ>1 using a fuel having wide ignition limits such as hydrogen (H2). A first lean range adjacent to the stoichiometric air ratio (λ=1) is defined and a second lean range following this first range in the direction of larger λ values is also defined.
Internal combustion engines, in particular hydrogen-operated internal combustion engines, can be operated under extremely lean conditions at air ratios far higher than λ=2 over a wide power range, so the level of nitrogen oxide emissions is negligible. Even the crude exhaust has significantly lower concentrations of nitrogen oxides (NOx) in such a hydrogen-based operation in comparison with emissions of a hydrocarbon-based process following aftertreatment of the exhaust using a λ-controlled three-way catalyst. Only if a relatively high power output is required it is necessary to make the fuel-air mixture richer in the range of 1≦λ≦2, which then is associated with a drastic increase in NOx emissions and thus the need for aftertreatment of the exhaust.
Unthrottled and lean operation of the internal combustion engine over a wide power range offers definite advantages in terms of fuel consumption because efficiency is improved with a greater air excess and unthrottled operation.
To prevent problematical NOx emissions in the lean range close to λ=1, DE 195 26 319 A1, proposes always operating with an air ratio of λ<1 and passing the exhaust over a catalytic converter. This approach cannot be regarded as leading to the goal because the special advantage of engine operation that is extremely favorable from the standpoint of consumption with negligible NOx emissions at the same time is not utilized in the extremely lean range of λ>2.
In the case of bivalent internal combustion engines in particular, which can be operated with both hydrogen and hydrocarbons such as gasoline, diesel or natural gas, it is recommended that the exhaust aftertreatment technology required for hydrocarbon operation of the internal combustion engine also be used for hydrogen operation.
The conventional catalytic exhaust aftertreatment method with hydrocarbon-operated internal combustion engines includes three-way catalytic converters and/or NOx storage catalytic converters. It is a disadvantage here that the λ-controlled three-way catalyst converter is effectively capable of reduction of nitrogen oxide in principle only at a stoichiometric air ratio of λ=1. In the lean range at λ>1, its nitrogen oxide reduction behavior is negligible, so that use for lean concepts is out of focus. However, the reduction behavior of NOx storage catalytic converters, which can be used in principle with an excess of oxygen in the exhaust (λ≧1), is definitely inferior to that of three-way catalytic converters at a stoichiometric air ratio, so this technology is not adequate to take into account extremely high demands with regard to fulfilling extremely stringent exhaust emission regulations.
Therefore, the object of this invention is to provide a method for operating an internal combustion engine, in particular a bivalent engine, with hydrogen, whereby excess NOx emissions are effectively prevented even when a high power is demanded.
This object is achieved whereby according to the basic idea, the engine is operated essentially in the second lean range—not directly adjacent to the stoichiometric air ratio λ=1—or at an air ratio that is at least approximately stoichiometric (λ≈1). The first lean range adjacent to the stoichiometric air ratio (λ=1) is faded out so no operation in this range is provided.
Especially preferred refinements and embodiments of the inventive method are the subject of the dependent claims.
It is regarded as highly advantageous if the first lean range includes essentially air ratios (λ) with significant nitrogen oxide emissions (NOx emissions) and the second lean range includes essentially air ratios (λ) with negligible NOx emissions.
Expediently the first lean range is skipped through an appropriate design of the engine control, which also contains the formation of the fuel-air mixture, by increasing the supply of fuel and/or throttling and/or varying the rate of recirculated exhaust gas. Advantageously the use of exhaust gas recirculation offers the possibility of at least largely unthrottled engine operation with a corresponding efficiency advantage.
According to an especially preferred embodiment of the invention, the internal combustion engine is operated over a wide power range in the second lean range with an especially large λ, but when a very high power is demanded in particular, the internal combustion engine is operated at least approximately at a stoichiometric air ratio (λ≈1), with a sudden transition to the at least approximately stoichiometric air ratio (λ≈1) starting from the second lean range, thus skipping the first lean range.
It is very advantageous if there is a catalytic exhaust aftertreatment, in particular when operating at λ≈1, expediently using a three-way catalytic converter and/or an NOx storage catalytic converter.
In particular when using an NOx storage catalytic converter, rich operation at λ<1 is recommended at least briefly, depending on the storage capacity and/or loading of the NOx storage catalytic converter, for regeneration of same.
The nitrogen oxides (NOx) are preferably reduced by means of unburned hydrogen (H2) present in the exhaust.
An especially preferred exemplary embodiment of this invention is illustrated and described in greater detail below with reference to the figures, which show schematically and as an example.